US-led coalition aircraft have bombed a convoy of pro-Syrian government forces in a remote area near the border with Jordan, US and rebel officials say.
Syrian and Iranian-backed militiamen were reportedly moving towards the Tanf base, which is used by rebel fighters and Western special forces personnel.
The US-backed Maghaweer al-Thawra rebel faction said the convoy was 27km (17 miles) away and contained four tanks.
There was no immediate comment from the government or its allies.
Rebel officials said earlier this week that hundreds of troops and militiamen were advancing into the sparsely-populated desert region, in an apparent attempt to halt recent rebel gains against so-called Islamic State (IS).
A US defence official said the pro-government convoy had not responded to numerous warnings that it was “getting too close to coalition forces” stationed at Tanf.
A telephone call was put into the Russian military, which backs President Bashar al-Assad. That was followed by a “show of force” by aircraft overhead and warning shots, the officials said.
“Then there was finally a strike against the lead portion of that movement.”
Muzahem al-Saloum of Maghaweer al-Thawra told Reuters news agency that coalition jets had struck after rebel forces clashed with the convoy.
“We notified the coalition that we were being attacked by the Syrian army and Iranians in this point, and the coalition came and destroyed the advancing convoy,” he said.
The pro-opposition Palmyra News Network said a number of vehicles were destroyed and that there were casualties.
The US defence official told AFP that the strike did not signal a strategic shift for the coalition, which has focused on battling IS and has tried to avoid intervening in Syria’s civil war.
US-led troops helped repel a major IS attack on Tanf last month, while Russian warplanes bombed the base in June 2016.
Syria war: US-led coalition ‘strikes pro-Assad convoy’}